Continent-continent Collision Zone Research Articles

A Lithospheric Cross-Section Through the Swiss Alps-I. Thermokinematic Modelling of the Neoalpine Orogeny

In this paper we develop a forward 2-D thermokinematic model to investigate the Neoalpine 35-0 Ma phase of orogeny along the European Geotraverse (EGT) through the Swiss Alps on a crustal and lithospheric scale. Using a divergence-free kinematic model (div v=0), we define mass displacements, which subsequently serve as input to a transient thermal model. the thermal model uses critically assessed material prorameters and accounts for the depth dependence of the thermal properties in processes such as crustal thickening and mantle-lithospheric subduction. Based on the presentday density pattern of the deep seismic image and estimated exhumation and shortening rates, we derive, in a first modelling step, a mass-displacement field describing the Neoalpine orogeny as a uniform process in time. In a second—thermal—modelling step, this kinematic scenario is further refined by modelling the non-uniform cooling histories of the southern Lepontine in the Penninic domain. For that purpose we adopt lithospheric shortening rates—and consequently exhumation rates—to agree with total Neoalpine shortening, while keeping the geometry of the kinematic model fixed. the resultant thermokinematic model reflects the main characteristics of Neoalpine tectonics, and shows a good overall agreement with combined geological and geophysical data. the asymmetric feature of the present-day tectonic structure along the profile is strongly reflected in the thermal structure of the lithosphere. This demonstrates the need for a kinematic model to investigate the deep-temperature field in active tectonic provinces. For further refinement of the model, the amounts of shortening have to be more precisely estimated, and a higher spatial density in geochronological and metamorphic data is required. Furthermore, surface heat-flow values are, up to now, too uncertain to constrain the predicted surface heat flow. In summary, our results show that we need, in particular, data constraining the horizontal component of the tectonic and thermal evolution. the results of the Neoalpine orogeny modelling demonstrate that the presented thermokinematic procedure yields a good first-order approximation to investigate crustal-scale and lithospheric processes. We conclude. therefore, that the approach presented provides the potential for application not only to continent-continent collision zones, but also to any active tectonic province.

Three-Dimensional Numerical Modeling of Deformation and Stress in the Himalaya and Tibetan Plateau with a Simple Geometry.

The Himalaya-Tibetan region is the most active continent-continent collision zone in the world. To understand the tectonics of the region, we have numerically modeled the deformation and stress fields in the region by using a three-dimensional finite element technique. For the sake of simplicity, our model space has a simple geometry. It covers a fan-shaped region including the Himalayan Arc of about 2, 000 km in length in the south and the Tibetan Plateau in the north, and extends to a depth of 250 km. We consider several combinations of boundary conditions. In a typical case, the boundary conditions are as follows: on the southern boundary of the model space, we impose a uniform velocity of 50 mm/year northward in the direction normal to the Himalayan Arc; the northern and basal boundaries of the model space are constrained by roller conditions; and other boundaries of the model space are left free. We do not consider the gravitational forces. In this case, the deformation field shows that the region far inside the Tibetan Plateau is experiencing an east-west extension at a rate of 6 mm/year and that the maximum uplift at a rate of -4 mm/year occurs mostly along the Indus-Tsangpo suture zone. The stress field has interesting features; the magnitudes of the near-surface stresses beneath the Lesser and Greater Himalaya varies considerably along the arc. It is large in the western and eastern Himalaya (the maximum rate of stress build-up is - 0.003 MPa/year) and small in the central Himalaya. In the Tibetan Plateau, the stresses are quite large in the central portion. The modeled stresses far inside the Tibetan Plateau agree with strike-slip faulting predominantly observed there, while those in southern Tibet are not consistent with normal faulting revealed by focal mechanism solutions.

Tertiary Tectonic and magmatic evolution of western Sulawesi and the Makassar Strait, Indonesia: evidence for a Miocene continent-continent collision

Abstract New field and laboratory data from western Sulawesi, Indonesia, integrated with available data establish its Late Cenozoic igneous framework and a new model for its tectonic evolution. Western Sulawesi contains three major Neogene N-S-trending tectonic domains (from W to E): (1) an active foldbelt, in which Pliocene and Miocene volcanogenic rocks are involved in W-vergent thrusting which extends into the Makassar Strait; (2) a central belt comprised of a deformed submarine Miocene volcanoplutonic arc built on an Oligocene-Eocene clastic and carbonate platform with Latimojong Mesozoic basement metamorphic and sedimentary rocks thrust over its eastern margin on W-vergent faults; and (3) an accreted Cretaceous-Palaeogene(?) ophiolite (Lamasi Complex) between the Latimojong basement block and Bone Bay. The Lamasi Complex ophiolite includes dioritic plutons, basaltic sheeted dykes, pillow lavas, greenstones, tuffs and volcanic agglomerates with depleted (MORB-like) Sr & Nd isotope and REE characteristics of probable normal oceanic crust with possible subduction-related or back-arc affinity. New K-Ar, 40 Ar- 39 Ar, Rb-Sr, and Nd-Sm isotope data suggest Cretaceous to Eocene crystallization and Oligocene to Miocene obduction. Late Miocene to Poliocene extrusive and intrusive rocks form a cogenetic volcanoplutonic complex of calc-alkalic to mildly alkalic, potassic, and shoshonitic felsic and mafic magmatic rocks of bimodal composition which were erupted and intruded during a short episode of Middle Miocene to Pliocene (3–18 Ma) lithospheric melting. Based on new Rb-Sr, Nd-Sm, and U-Pb isotope, and major and trace element geochemical data, parental source rocks of the Miocene melts were Late Proterozoic to Early Palaeozoic crustal and mantle lithospheric assemblages which became heated and melted owing to a continent-continent collision in which west-vergent continental lithosphere derived from the Australian-New Guinea plate was subducted beneath eastern-most Sundaland. The timing of this magmatism and subsequent cooling and denudation history are constrained by 113 new K-Ar, 40 Ar- 39 Ar, and fission track ages. The new tectonic model differs significantly from previous models: the Makassar Strait is now interpreted as a foreland basin bound on both sides by converging Neogene thrust belts, in contrast to previous models suggesting Late Tertiary oceanic spreading or continental rifting. West-vergent obduction of a pre-Eocene oceanic, primitive arc, or back-arc crust onto western Sulawesi occurred during late Oligocene to Miocene times. The Late Miocene western Sulawesi magmatic arc is envisioned as a continent-continent collision product, in contrast to previous models involving a normal ocean-continent or ocean-ocean subduction-related magmatic arc (west or east vergent) or post-subduction rifting. The east Sulawesi ophiolite extends into western Sulawesi, suggesting that Bone Bay resulted from collapse of the over-thickened Miocene orogen. The new tectonic model illustrates the central role western Sulawesi plays in unlocking the complex evolution of Indonesia as well as the temporal and magmatic details of a continent-continent collision zone.

Three-dimensional collisional structure of the Trans-Hudson Orogen, Canada

The three-dimensional structure of the eastern Trans-Hudson Orogen (THO), part of a Paleoproterozoic continent-continent collision zone in central North America, is revealed through a network of LITHOPROBE seismic reflection profiles. The seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust. E-W profiles show strong, E-dipping reflections extending throughout the crust while N-S profiles record events outlining antiformal and synformal structures. This allows the identification of décollements that may have localized along pre-existing structures (e.g. possible basin-bounding extensional faults) and at major rheological boundaries (e.g. basement-cover contact, upper-middle crust transition). The present topographic surface displays oblique crustal sections with 10–15 km of structural relief, generated during post-collisional, intracontinental transpression of THO as a result of crustal-scale folding and faulting.

Structure of a Paleoproterozoic continent-continent collision zone: a LITHOPROBE seismic reflection profile across the Trans-Hudson Orogen, Canada

An ~ 800 km reflection seismic profile across the Trans-Hudson Orogen, northern Saskatchewan and Manitoba, images crustal-scale tectonic imbrication in an unprecedented picture of Paleoproterozoic crustal accretion and continent-continent collisional tectonism. The profile is crudely symmetric about a crustal-scale culmination in the western part of an accreted juvenile collage (Reindeer Zone). Geologic and isotopic data suggest that this culmination is cored by microcontinental Archean basement. West of the culmination, highly reflective juvenile crustal elements dip westward into the lower crust, beneath the Wathaman Batholith and Archean continental crust of Hearne craton. To the east, strong reflections in the juvenile Reindeer Zone crust and reworked Archean foreland of the Thompson belt have eastward dips persisting into the middle crust and extending beneath the Superior craton. A continuous reflection Moho, well-defined for > 500 km in the western part of the profile, shows marked relief (12- > 15 s), including a prominent root below the crustal culmination. These imaged structures give evidence of substantial crustal shortening and thickening via large-scale imbrication consistent with collisional orogeny. W-dipping structures below the Wathaman Batholith and reworked Hearne craton may reflect subduction polarity in this part of the orogen. However, geological evidence suggests that E-dipping structures below Superior craton are largely related to late/post-collisional deformation, rather than to prior oceanic subduction polarity.

Demagnetization by hydration in deep-crustal rocks in the Grenville province of Quebec, Canada: Implications for magnetic anomalies of continental collision zones

Continent-continent collision zones of various ages are characterized by low magnetic susceptibilities and hence weak intensities of induced magnetization; hydration reactions in deep-crustal rocks from the Grenville province of the Canadian Shield may offer an explanation for the weak induced magnetization of these zones. Troctolite with an oxide assemblage of Cr-bearing magnetite + Cr-Mg-bearing hercynite + ilmenite and magnetic susceptibilities of ∼10 -3 to 10 -2 (SI units) has been converted by hydration and deformation to a rutile-bearing plagioclase-amphibole-garnet gneiss with susceptibilities of ∼5 x 10 -4 . The complete transformation of troctolite to gneiss is essentially isochemical except for H 2 O content and occurs over 1-2 m in the field. The order-of-magnitude change in magnetic susceptibility is even more abrupt. This susceptibility contrast corresponds to decomposition of the oxides of the troctolite (particularly Cr-bearing magnetite), liberating Fe, Al, Cr, and Ti to be incorporated in amphibole and rutile. Pervasive deformation of the lower crust in continental collisions may be accompanied by widespread hydration and the destruction of magnetic minerals, explaining why many continental collision zones have weak intensities of induced magnetization.

The Palaeozoic history of an unusual intracratonic thrust belt in central Australia based on 40 Ar- 39 Ar, K-Ar and fission track dating

Intraplate thrusting, involving reactivation of a mid-Proterozoic province boundary within the basement metamorphic rocks of the Arunta block, occurs at the northern margin of the late Proterozoic to mid-Palaeozoic Amadeus Basin in central Australia. Earlier geological and geophysical studies show that the thrust system, referred to as the Alice Springs orogen, is 'thick-skinned' in the central part of the thrust belt and was dominated by the crust-cutting Redbank thrust zone. Several episodes of thrusting, defined stratigraphically, began about 400 Ma ago. Analysis and modelling of 40 Ar- 39 Ar age spectra on K-feldspar from the basement rocks, combined with regional geological constraints, point to cooling rates of 3–10 °C/Ma and moderate overall exhumation rates of 100–400 m/Ma during the Alice Springs orogeny, without appreciably increased heat flow. However, final uplift along the thrust front at 300–320 Ma was rapid, as the time of final closure of small domains in K-feldspar approximates apatite fission track ages. By using 40 Ar- 39 Ar age data as a constraint, together with available geological and geophysical data, crustal shortening across the thrust belt is thought to be approximately 20 to 30%. Overall convergence rates of 0.6–3 km/Ma are slow compared with rates of ≥ 5 km/Ma for continent-continent collision zones.

A gravity model for the lithosphere in western Kenya and northeastern Tanzania

We present a new gravity model for the lithosphère beneath the Kenya Rift Valley, the Mozambique Belt, and the Tanzania Craton in western Kenya and northeastern Tanzania. The Kenya Rift lies within the eastern branch of the extensive Cenozoic East African Rift System and has developed almost entirely in the Pan-African Mozambique Belt about 50 to 150 km east of the exposed margin of the Archean Tanzania Craton. The gravity field over western Kenya and northeastern Tanzania is characterized by a long-wavelength Bouguer anomaly. We propose that this anomaly has two components: 1. (1) a “rift” signature, deriving from a shallow rift basin, a lower crustal intrusion and a low-density zone in the mantle lithosphere localized beneath the rift axis 2. (2) a “suture” signature, arising from a crustal root along the boundary between the Mozambique Belt and Tanzania Craton and higher density crust in the mobile belt above part of the crustal root. Two lines of reasoning support our interpretation: 1. (1) Recent geological studies of the Mozambique Belt in Kenya and Tanzania suggest that it is a continent-continent collision zone, and continent-continent collision zones worldwide commonly exhibit a characteristic gravity anomaly. 2. (2) The long-wavelength Bouguer anomaly has at least two minima, one over the craton-mobile belt boundary, and one or more over the rift valley. Corroborative evidence for our interpretation of the gravity field is provided by recent seismic investigations.

The late Archaean mobile belt through Godthabsfjord, southern West Greenland: a continent-continent collision zone?

In the Godthabsfjord region of southern West Greenland a NE-SW-trending belt of rocks of very varied age and origin, here named the Akulleq terrane, is separated by major faults from more extensive blocks of typical high-grade Archaean rocks that, although they are superficially similar, have different ages and metamorphic histories. The continental crust that forms the block to the north-west, the Akia terrane, was accreted between ea. 3200 and 2980 Ma, and that forming the block to the south-east, the Tasiusarsuaq terrane, between 2920 and 2800 Ma. It is suggested that the Godthabsfjord belt is the result of collision of the two continental blocks between 2800 and 2650 Ma. The rocks of the Akulleq terrane are interpreted as fragments of different parts of the crust that originally separated the two continents. They include early Archaean continental crust, possible oceanic crust, and acid to intermediate rocks of intrusive and possibly also extrusive origin that may have been generated in a subduction-related environment.

The granulite-eclogite facies transition: A comparison of experimental work and a natural occurrence in the Bergen Arcs, western Norway

A unique segment of the deeper parts of the Caledonian continent-continent collision zone is exposed in one of the thrust sheets in the Bergen Arcs of western Norway. It demonstrates that Precambrian granulite-facies rocks underwent fluid-controlled eclogitization on a regional scale. The transition from granulite to eclogite facies is sharp. It occurs over distances of centimetres to metres and follows shear zones and infiltration fronts. Major anastomosing shear zones, typically 50 to 100 m thick, with well-banded eclogite-facies rocks crosscut the lithological boundaries. They can be followed continuously for several kilometres. Breccias, where metre-size blocks of granulite float in an eclogite matrix, outcrop over areas of 0.5 km 2. Areas comprising more than 80% eclogite occupy several square kilometres. This mixture of metamorphic facies indicates the metastable occurrence of granulite-facies rocks in the lower crust of modern and ancient orogenic belts. In contrast to earlier experimental work, such field relations imply that the granulite-eclogite facies boundary in nature can be sharp and independent of lithological boundaries. A partly eclogitized lower crust, of the kind exposed in the Bergen arcs can explain the 7.2–7.8 km/s Vp layer present in many crust-mantle transition zones. Anastomosing shear zones and areas of almost complete eclogitization represent potential lower crustal reflectors.

Regional compensation of the Greater Caucasus mountains based on an analysis of Bouguer gravity data

Teleseismic and geological observations of the continent-continent collision zone in the Greater Caucasus mountains of the southwestern Soviet Union indicate that the range is presently being underthrust primarily from the south. Since the onset of Alpide deformation, tectonic activity has been concentrated along the southern slope of the mountains, but ongoing deformation along the northern slope in the eastern part of the range may be associated with thrusting of the mountains northward over the Scythian platform in this area. To understand the present-day plate interactions in the Caucasian region and the mode of compensation of the Greater Caucasus mountains, we analyze Bouguer gravity anomaly data using both spectral (coherence and admittance calculations) and traditional spatial domain forward modeling techniques. As a whole, the range appears to be in regional isostatic equilibrium with compensation by flexure of an elastic plate of approximately 40 km thickness ( D = 6×10 23Nm) loaded strongly from the bottom as well as the top. Subsurface loads corresponding to stacking of thrust sheets, uplifts of basement rocks, active volcanic zones, and thick sequences of basin sediments play an important role in improving the fit between observed and calculated Bouguer values. Closer examination of the individual tectonic provinces which make up the range yields results consistent with support by a 40–50 km thick plate in the eastern and central parts of the Greater Caucasus mountains and by a plate with no flexural rigidity in the western part of the range. Coupled with the active volcanism and the relative lack of seismicity in the western part of the Greater Caucasus region, these results indicate the presence of hot, thermally-altered lithosphere in this area contrasting to the colder, more brittle, and more elastic lithosphere of the seismically active eastern region.

Geochemical characteristics of collision-zone magmatism

Summary This paper reports the results of a systematic geochemical study of intermediate and acid intrusive rocks from a number of continent-continent collision zones of Phanerozoic age. Four groups of intrusions can be recognized, each associated with a particular stage in the tectonic evolution of a collision zone. Pre-collision calc-alkaline (volcanic-arc) intrusions which are mostly derived from mantle modified by a subduction component and which are characterized by selective enrichments in LIL elements. Syn-collision peraluminous intrusions (leucogranites) which may be derived from the hydrated bases of continental thrust sheets and which are characterized by high Rb/Zr and Ta/Nb and low K/Rb ratios. Late or post-collision calc-alkaline intrusions which may be derived from a mantle source but undergo extensive crustal contamination and can only be distinguished from volcanic-arc intrusions by their higher ratios of Ta/Hf and Ta/Zr. Post-collision alkaline intrusions which may be derived from mantle lithosphere beneath the collision zones and which carry high concentrations of both LIL and HFS elements. The geochemical evolution of crustal melts within groups (ii) and (iii) can be viewed in terms of the dehydration reactions, volatile transfer and transient geothermal gradients that result from thrust tectonics in the zone of collision.

紀伊半島中部，大峰地域の中新世ＩタイプおよびＳタイプ花こう岩質岩の微量化学組成

The Ohmine granitic rocks of Miocene age occur in the central Kii peninsula, Outer Zone of Southwest Japan. These rocks fall into two distinct types; S-type and I-type (Dorogawa and Shirakura rocks). Trace elements (Ba, Ce, Co, Cr, Cs, Nb, Ni, Rb, Sc, Sr, Y, Zr, and F) of the rocks were determined by photon activation, colorimetry, and ion-electrode analyses. The S-type granitic rocks are high in SiO2, F, and Rb/Sr and low in Sr compared with the I-type ones. The Shirakura rocks formed early stage in the intrusive sequence are higher in D. I., Ce, Rb, and Nb and lower in MgO/(MgO+FeOt), Co, Cr, Ni, and Sr than the Dorogawa ones. The S-type magma, from which biotite, plagioclase, and orthoclase must have fractionated, would be formed by partial melting of biotite and orthoclase containing sedimentary and/or metasedimentary rocks at about 700°C and 5 kbar. The I-type granitic rocks were probably generated by partial melting of Caamphibole and plagioclase containing intermediate igneous and/or metaigneous rocks. Fractional crystallization mainly of plagioclase, Ca-amphibole, and biotite must have occurred in the I-type magma. The Shirakura magma would be produced by lower degree of partial melting than the Dorogawa one. The crystallization course of the I-type magma was different from that of the S-type one ; orthoclase precipitated next to quartz in the I-type magma, on the contrary quartz crystallized after orthoclase in the S-type magma. On the basis of trace elements behavior, it is unlikely that the granitic rocks are products of large scale magma-crust interaction and of mixing between basaltic and granitic magmas. On the criteria of alumina saturation and isotope ratio, revised classification of calc-alkali granitic rocks are proposed; C-type (sub to metaluminous, SrIo〉 0.708) and W-type (per. aluminous, SrIo〈0.708) in addition to I-type and S-type. C-type rocks are, however, not found yet. The parental materials and fractionating phases of the I-type, W-type, and the S-type in the world are estimated from publicated data. The estimated parental materials and fractionating phases of the I- and W-type and the S-type are the same as those of the Ohmine I-type and the Ohmine S-type, respectively. The I- and W-type rocks are, therefore, in lower range of Rb/Sr than the S-type ones. Many I- and W-type granitic rocks occur in mature continental regions composed of basaltic and granitic layers. The S-type granitic rocks would, on the other hand, occur in the regions consisting of immature crust without basaltic layer (e. g. southern part of the Outer Zone of Southwest Japan), and in the regions of crust exceeding 50km in thickness such as continental margins (e. g. western Bolivia) and continent-continent collision zone (e. g. High Himalaya).